6-26-16 Seung-Ho Joo and Tae-Hwan Kwak write in their book Korea in the 21st Century: "South Korea may find economic benefits in military cooperation with Russia. Seoul and Moscow can jointly develop advanced technology and high-tech weapons and sell them in the world market. The relationship between the two countries may be mutually complementary: Russia has two advantages in basic sciences and advanced technologies, while South Korea has strengths in marketing skills and capital."

The S-400 isn't cheap – it costs around $500 million per battalion. That's why only countries with deep pockets like China and India (which is negotiating a deal) will be fielding this weapon. But to the credit of the South Koreans, they beat both the Chinese and the Indians to this prized weapon by more than a decade. http://www.upi.com/Business_News/Security-Industry/2016/06/28/S-400-technology-helps-Seoul-blunt-Pyongyang-threat/8111467123421/?spt=hs&or=si

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4-16-16 Russians, however, were offering their very latest technology – yes provided that South Korea makes its purchases in meaningful quantity but without tying transfers to mega-million deals.

Thus the Korean military drew a list of what it wanted the most: 33 T-80U tanks, 33 BMP-3 infantry fighting vehicles, over a thousand anti-tank "Metis" missile launchers and dozens of anti-aircraft "Igla" missile launchers. All were dutifully delivered in 1995-96.

Now one may ask how come? How come Russia was willing to arm South Korea with tanks that far outclassed anything its traditional all North Korea had? And how come it was so cavalier about transferring its latest in tank technology to a close US ally?

The answer to the first question is that in the 1990s Moscow had essentially abandoned North Korea as it instead courted the cash-rich South, in much the same way Seoul (for different reasons) courted the Soviet Union in the 1980s.